This invention relates to an improved method and apparatus for controlling the energy level of a laser beam used in a materials processing machine.
Manufacturing and materials processing machines that use lasers for their source of processing energy have had little need for extreme accuracy in controlling the laser beam energy. In some resistor trimming machines, for example, a laser beam is used to ablate a resistor material while not unduly damaging the surrounding materials. There is a range of laser energies suitable for this process. The laser energy must be sufficient to cause removal of the material but must not exceed that which would cause undue damage to the surrounding materials. Within this range, some predetermined intermediate energy can be selected. However, in other applications, the energy of the laser beam must be carefully controlled. For example, techniques have been developed for forming very accurate, thin conductive lines on a semiconductor device. This is achieved by crystallization of amorphous silicon by a laser beam, as disclosed in copending U.S patent application Ser. No. 07/213,146, filed June 29, 1988, and which is assigned to the common assignee herewith. In such a technique, the amorphous silicon is crystallized by the laser beam to form a path upon which the conductive material can be deposited. If the path or line width grows too large because the energy density is high or too narrow because the energy density is low, unacceptable results will be obtained. It is, therefore, important that the laser beam energy be accurately controlled.
Although a laser beam generating system is set up to maintain constant energy during use, it is very difficult to determine and accurately control the energy density. This happens because of thermal drifts in the laser system, misfocusing, heating of some of the optical components thereof, and because the laser power level of laser systems varies a small amount with time in a random manner. Also, when used to crystallize amorphous silicon, the exact optical characteristics of the amorphous silicon varies depending upon its deposition conditions and the underlying substrate surface.
Accordingly, there is a need in the art for an improved method and apparatus for controlling the energy level of a laser beam. It is desirable that such a method and apparatus be automated and achieve an energy level that is optimum for the particular process.